"This has been a difficult personal decision, and I am very grateful to my family for their total support of my going forward, had that been what I decided," the Mississippi governor said in a statement.

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"A candidate for president today is embracing a 10-year commitment to an all-consuming effort, to the virtual exclusion of all else," Barbour added. "His (or her) supporters expect and deserve no less than absolute fire in the belly from their candidate. I cannot offer that with certainty, and total certainty is required."

Barbour's announcement stunned political insiders — just hours earlier, many expected him to meet his self-imposed deadline and enter the race by the end of April. The decision was "surprising to say the least,” said GOPAC chairman and longtime Republican insider Frank Donatelli. “He would have been a very credible candidate with a strong record and access to campaign funds. ... His withdrawal leaves a vacuum for someone else."

Barbour had recently traveled to Iowa, New Hampshire, South Carolina and Nevada — all four early-voting states — attracting large followings and ample media attention at most stops. During the campaign-style visits, Barbour told potential supporters that he was "thinking about running for president" — but few observers believed he was actually on the fence.

Political watchers were quick to speculate that Barbour's move may increase the likelihood that Mitch Daniels — Indiana governor and Barbour's close friend — will indeed run, despite little recent campaign activity on his part. Both had indicated in the past that they did not want to run against each other. But Daniels has continued to sound wary about the race and would face many of the same challenges that Barbour would have encountered — including a spouse who is not eager to embark on a national campaign.

Barbour aides didn't have anything to add about his decision. "He decided not to run" is all nephew and political hand Henry Barbour said. A Barbour spokesman declined to comment.

Another Barbour confidant offered only that “he's an incredibly smart guy who is going to be a huge presence in the party for a long time. While this decision takes out an experienced conservative candidate, I still think we have a great shot at defeating President [Barack] Obama.”

The announcement comes a week after a fruitful trip to South Carolina, in which Barbour captured the Charleston County GOP straw poll, commanding 22 percent among the small sample of Republican activists.

But his decision also followed waves of negative media attention for his past as a lobbyist and statements that many saw as insensitive on racial issues. Though Barbour tried to deflect attention from previous controversial quotes and embrace his lobbying past head on — arguing bluntly that lobbying is what presidents do — those two issues have frequently dogged the Mississippi governor. His decision may stem, in part, from a perception that the ex-lobbyist and consummate insider could be more effective off the stage than on it.